1. Field of the Invention
The present invention is directed to a method for the implementation of a dynamic magnetic resonance measurement with a contrast agent, of the type wherein at least one imaging magnetic resonance measurement of a subject region is implemented using an inversion recovery sequence at a first point in time and possibly further points in time, after administration of the contrast agent in order to obtain a magnetic resonance image of the subject region.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for acquiring images of the inside of the body of a living examination subject. For implementation of magnetic resonance tomography, a basic field magnet generates a static, relatively uniform basic magnetic field. Rapidly switched gradient fields that are generated by as gradient coils are superimposed on this basic magnetic field during the exposure of magnetic resonance images. Radiofrequency pulse sequences are emitted into the examination subject with radiofrequency transmission antennas for triggering magnetic resonance signals that are picked up by radiofrequency reception antennas. Magnetic resonance images with different weightings of individual tissue types or substance types of the subject region of interest can be generated with different pulse sequences.
A field of magnetic resonance tomography relates to dynamic magnetic resonance measurements with the use of contrast agents. Vitality (viability) examinations at the heart after a heart attack or a surgical intervention at the heart is one example of this field. The contrast agent injected into the patient causes a temporary modification of the T1 times, i.e. the longitudinal relaxation times, of the tissue. A viability examination exploits the fact healthy tissue (with blood circulation) absorbs the contrast agent quickly and also releases it quickly, whereas the contrast agent increases in concentration relatively slowly in dead tissue. In a dynamic measurement, a waiting time of, for example, 5 minutes is therefore introduced after injection of the contrast agent before a first imaging magnetic resonance measurement of the subject region of interest is implemented. As a rule, a number of such measurements are implemented at further time intervals, so that the so-called late enhancement in dead tissue regions can be observed on the basis of the magnetic resonance images produced at different points in time. Dynamic magnetic resonance measurement with the application of contrast agent thus enables the identification of dead tissue regions affected by the cardiac infarction that can no longer be revitalized by means of a surgical intervention. The success of a surgical intervention, for example a percutaneous transluminal coronary angioplasty (PTCA) or a coronary bypass operation (CABG), also can be reviewed in this way.
Fast gradient echo sequences are utilized in these measurements in order to avoid motion artifacts. A technique known as the inversion recovery technique can be utilized for generating high-contrast magnetic resonance (MR) images, wherein the longitudinal magnetization is inverted with a 180° pulse and interrogated with a further RF pulse after an inversion time T1. The inversion of the longitudinal magnetization by the 180° pulse also is referred to as a preparation phase for the measurement. The inversion time T1 represents a significant factor for the generation of high-contrast MR images. With a suitable selection of this inversion time, the healthy muscle tissue (myocardium) appears dark in the MR image, whereas the region enhanced with contrast agent are displayed light (bright). Due to the time intervals between the individual measurements, however, the T1 times of the healthy muscle tissue change because the contrast agent is flushed out, so that a different T1 time must be employed for an optimum contrast at each measurement time of the dynamic measurement.
The selection of suitable T1 times conventionally has been based on empirical data. The T1 time is therefore iteratively adapted from measurement time to measurement time over the course of the examination. Conventionally, the T1 time has to be followed-up according to the empirical values given repetitive repetitions of such measurements, for example, up to a time of 300 ms in steps beginning with a start value of T1=260 ms for the myocardium suppression. This empirical T1 selection, however, does not always achieve an optimum contrast with a suppression of healthy muscle tissue in the MR image of the heart on a case-by-case basis.